X-ray diffraction method



w PARRISH ET AL 2,549,987 X-RAY DIFFRACTION METHOD Flled March 27 1948Pl YZZMMPABEZSH V EWABDAJZWJMCHEB INVENTORS BY%% ANT A ril 24, 1951Patented Apr. 24, 1951 UNITED STATES PATENT OFFICE 2;549,9s7" X-RAYDIFFRACTION METHOD William Parrish, Hastings, and'E'dward A. Ham

acher; Irvington; N. Y., assignorstoPhilip's Laboratories, Inc.,Irvington on Hudson, N. Y. Application March--27, 1948-, Serial No.17,398- 6'-Clairns; (o1; 256-52) The invention relates to a methodforanaly' zing a crystalline material by X-ray' diffraction. Moreparticularly, the invention is directed to: a method of crystallographicanalysis in which an incident beam of X-radiation is reflected from acrystalline specimen and angles of reflection are measured.

In the more specific sense, the invention is 'directed to an improvementin the Bragg Focussing technique which is Well known in the art of.X-ray diffraction studies and is fully described in the Spectroscopy ofX-rays b'y Siegbahn (the Eng'lish translation by Lindsay, published bythe Oxford Press, 1925, page 20) in the Chemical Analysis by X-rays andIts Applications by Georg von I-IeVesy, published by MacMillan, 1932, inpages 22 et sequi'tur; and in Compton and Allison, X-rays in Theory andExperiment, page 684, published by Van Nostrand. A detailed discussionof the Bra'gg'focussing method is therefore believed unneces saryfor'the proper understanding of the invention.

However, the Bragg focussing method as described in any ofthe referenceshas certain inherent limitations which the present invention overcomes'In particular, it is known that there is an apparent discrepancyintroduced in the measurement of the Bragg" reflection angle as a resultof a too large divergence of the incident and reflected beams ofradiation directed at and reflected from the specimen. If thedivergenceof the beam is limited in order to correct the discrepancy, theintensity of the beam is markedly reduced requiring longer exposuretimes. I

With conventional focal spot arrangements, the Bragg focussing techniquerequires a beam of large divergence in the plane of the specimen axis inorder to obtain sufficient intensity in thebeam reflected by thespecimen for energizing the detector, e. g., a Geiger-Miiller tube. Thislarge divergence thereby introduces a subsequent broadening of thereflected beam which results in the introduction of an error inthemeasured angle of the reflected beam, the Bragg angle and a decrease inthe resolving power of the instrument.

It is therefore an object of the inventionto provide a method employingthe Bragg focussing technique which eliminates errors in the measurementof the Bragg angle, 0.

It is a further object of the invention ;toprovide a method employingthe Bragg -focussing technique Whereinthe divergence in the plane of thespecimen axis is reduced without sacrificing-the intensity of the X-raybeam.

t It is a still rurtrierobject of the invention to provide a methodemploying the Bragg focussing technique in which the intensity of thereflected beam is increasedlwithout sacrificing'resolution.

These and furtherobjects of. the invention will appear from thedescription that follows.

According to the invention, a focal spot on the anode of an X-ray tubeis viewed at very' small angles along the Width dimension in order toobtain a narrow source of radiation. For ex; ainple, a focal spot 9 by2',mms. wide is vie'wed along the 2mm. dimension at aniangle ofapproximately 3 with the anode: surface whereby an efiective focal spothaving the dimensions 9 by 0.1 mms. ,is'obtained whereas by viewing thefocal-spot along the length'din ension for example, at 3 the focal spotwould appear to have dimensions of .24 by '2 mms. wide) We prefer toemploy a long, thin focal spot parallel to the specimen axis and whichapproaches a' near line source. For ordinary Bragg focussing, thedivergence in the X-ray' beam in the plane of'the specimen axis wouldimpair resolution in the beam reflectedfrom the Specimen. However, inthemethodaccording' to the invention, we are able to use this focal spotarrangemengane avoid excessive divergence in the plane of the axis ofthe specimen. I

B wee t feca s o and, h s ec m ng ieterpose a l imat tem. 9 i in 9.f aplurality of thin parallel sheets oriented length;- Wise along thebeam'of radiation and perpen dicular .to the specimen axis. Thisarrangement of sheets is commonly knownas 'Soller slits and hereinafterWe propose to refer to this collimating system as Seller slits.

The-purpose of the Soller slits is in eiiect to break up the thin, longsource of radiation into a series of focal spots, diverging effectivelyonly in a plane perpendicular to the specimen axis, and having a limiteddivergence in" the plane of the specimen axis. Thus,- What we havedoneis to reduce a wide, diverging beam of radiation, having a.divergence both in the plane of the specimen'axis and inaa planeperpendicular to the specimen axis into a seriesof parallel beams havinga limited divergence in the plane of the specimen axis.

The resulting collimated beam after striking thespecimen is reflected atthe Bragg angle in the well-known manner and is thereafter 'collimatedby a'second' system oflSoller slits which similarly limit-the divergence'IOf' therefiec'ted beam in the plane'of the specimen axis. The beamreflected at the Bragg'angle after passing through the second systemfofSolier'islitsis selected by a receivingsli-t' parallel to the axis" oftwo stands.

ly understood and carried into effect, it will now.

be described with reference to the accompanying drawing in which:

Fig. 1 illustrates a preferred embodiment of an apparatus employing the,method according to the invention and is shown in perspective view withthe X-ray tube partially in section;

Fig. 2 is a perspective view 'of the geometry of the X-ray beam in avertical plane illustrating the method according to the invention.

Referring to the drawing, a vertically mounted Xraytube In, (Fig. 1) inwhich the elements are ar anged so'that the cathode I2 is parallel tothe face of anode and produces a focal spot-20" lengthwise across thetarget face when viewed from the specimen [5. As is customary, the focalspot is viewed at low angles 1 to 3, and preferably at an angle of 3with the'anode surface'sothat the focal spot appears effectively as a?thin long line of radiation having the di- 4 to the projected width ofthe focal spot by a receiving slit I9 mounted for convenience inmeasuring the Bragg reflection angles on a goniometer also mountedvertically and detected by Geiger-Muller tube 2|.

'The particular arrangement described is preferred because the basicgeometry of the system permits reflection angles 'to be measured in theback reflection region, i.-e. beyond 90 but the invention equallyapplicable to horizontally mounted tubes and goniometers wherein in theangles in the back-reflection region cannot be mensions 9 mms. long by0.1 mms. wide and i "emitsa diverging cone of radiation (shown-insection' in Fig. 2); Between specimen l5 and the-"tube window, andpreferably at the tube" window, a Soller slit system is arranged tolimit the divergence of the beam in the plane of the specimen axis l6 ascan be seen in Fig. 2. As can also be seen, the original diverging beamis split into a plurality of parallel beams of radiation each emanatingfrom a near point source on the anode; each 'beam diverging only in theplane perpendicular to the specimen axis. There wi-llbe a slightdivergence in the plane of the 'specimen axis of each beam afteremerging from 'the 's'oller'slits, butthis divergence isinconseq'u'ential since it is measurably below that ob stained.withordinary Bragg focussing without Soller'slits and with a near pointsource of radiaf i 1 After emerging from the Soller slits, the beam iseffectively a series of parallel beams of radia tion diverging in theplane perpendicular to the plane of the specimen axis. However, in orderto limit the width of the total beam in the plane perpendicular to theplane of the specimen axis to-the width of the specimen; a divergenceslit [4 is provided. 7 i

- 'Ihe specimen is mounted to rotate about its axis 16 in order tomeasure Bragg reflection angles which axis is parallel to the focalspot. Any conventional mounting arrangement which permits rotation ofthe specimen is suitable and the specimen is merely shown mounted in aholder'arranged to rotate on a shaft carried on After striking thespecimen, the beam is reflectedat the Bragg angle into a second Sollerslit system l8 arranged for rotation about'the axis of the specimen. TheSoller slit system is carried on an arm I! which is driven from theshaft carrying the specimen holder, and must be arranged to rotate at anangular speed twice that of the specimen. This second Soller slit systemcollimates the reflected beam in the same manner as the direct beam ascan be clearly seen from Fig.2of the drawing. 7 7

After emerging from the second Soller slit system, the vreflected beamis limited in width conveniently measured.

As can be clearly seen, the invention overcomes the inherent limitationof the ordinary Bragg focussing method in that excessive divergence ofthe beam in the plane of the specimen axis is avoided. This excessivedivergence has resulted in poor resolution in the reflected beam andfalse indications of the Bragg reflection angle. With our invention,these twin difficulties are obviated and the results have indicated fromcareful laboratory studies that there is a marked improvement in theresolution in the reflected beam and a truer indication. of Braggreflection angles. 7

However, we have found that in'order to avail all the advantagesinherent in thismethodxit is desirable to and necessary to employ" asuit able detection instrument. For the purpose if the invention we havefound a'Geiger-Miil'lfer tube employing a mica window with a cl'ilorineargon filling both necessary and desirable. The mica window reduces thedead end volume and window loss to a minimum thus increasing theefficiency of the tube; the argon is suitable as a filling for the tubefor high sensitivity and the chlorine increases the diameter of thesensitive region to utilize the full-length of the focal spot. Inaddition we have found it preferable to em ploy a mica window for theX-ray'tube.

Since the divergence o'f the beam" in the plane of the specimen axis isreduced byjfcollimatihjg the beam into a plurality ofbeams in the Sollerslit, the total beam has a higher intensity than thatordinarily used inBragg focussing.tech-; nique. Hence, as result of the higher'intensity;increased precision is obtained for X-ray diffraction studies to be madeon crystalline powder specimens. 7 P L i g While we have illustrated ourinvention in. a preferred embodiment of the apparatus, other forms ofapparatus and variations of the method are within the skill and judgmentof those skilled in the art without departing from the spirit and scopeof our invention.

What we claim is: a

1. A method of analyzing a crystalline material comprising the steps ofgenerating a beam of X-radiation from a line source of X-radia tion ofgiven intensity, rotating a specimen'of crystalline material about agiven axis passing through the specimen and parallel? to the line 7 ingthe specimen to obtain a reflected beam of radiation therefrom at agiven'angle of reflection with a plane perpendicular to the surface ofthe specimen, limiting the divergence of the reflected beam in the planecontaining the axis'of rotation, and detecting the reflected beam ofradiation.

2. A method of analyzing a crystalline material comprising the steps ofgenerating a beam of X-radiation from a line source of X-radiation ofgiven intensity, rotating a specimen of crystalline material about agiven axis passing through the specimen and parallel to the line sourceof radiation, collimating the beam of X-radiation while limiting thedivergence of said beam in a plane containing the axis of rotation ofthe specimen to produce a plurality of beams of radiation of likeintensity as said line source of radiation, impinging the collimatedbeam of limited divergence onto the specimen while rotating the specimento obtain a reflected beam of radiation therefrom at a given angle ofreflection with a plane perpendicular to the surface of the specimen,collimating the reflected beam into a plurality of beams while limitingthe divergence of the reflected beam in the plane containing the axis ofrotation, detecting the refiected beam of radiation, and measuring theangle of reflection of the reflected beam from the specimen.

3. X-ray diffraction apparatus comprising an Xray tube for generating aline source of X radiation, a first collimating system comprising aplurality of thin long parallel sheets impervious to X-radiation mountedperpendicular to said line source to collimate the beam of radiationinto a plurality of parallel beams of radiation, means to mount aspecimen of crystalline material for rotation about a given axis passingtherethrough and parallel to the line source of radiation to reflect abeam of radiation therefrom, a second collimating system mounted forrotation with said specimen and comprising a plurality of thin longsheets impervious to X-radiation which are mounted perpendicular to saidaxis of rotation, and means to detect said reflected beam of radiationmounted for rotation with said specimen and said second collimatingsystem whereby the reflected beam is detected and the intensity thereofdetermined.

4. X-ray diffraction apparatus comprising an X-ray tube for generating aline source of X- radiation, a first collimating system comprising aplurality of thin long parallel sheets impervious to X-radiation mountedperpendicular to said line source to collimate the beam of radiationinto a plurality of parallel beams of radiation,

means to mount a specimen of crystalline ma-' terial for rotation abouta given axis passing therethrough and parallel to the line source ofradiation to reflect a beam of radiation therefrom, a second collimatingsystem mounted for rotation with said specimen and comprising aplurality of thin long sheets impervious to K- radiation which aremounted perpendicular to said axis of rotation, means to detect saidreflected beam of radiation mounted for rotation with said specimen andsaid second collimating system whereby the reflected beam is detectedand the intensity thereof determined, and means to measure the angle ofreflection of the reflected beam from the specimen.

5. X-ray diffraction apparatus comprising an X-ray tube for generating aline source of X- radiation, a first collimating system comprising aplurality of thin long parallel sheets impervious to X-radiation mountedperpendicular to said line source to collimate the beam of radiationinto a plurality of parallel beams of radiation, means to mount aspecimen of crystalline the projected width of the line material forrotation about a given axis passing therethrough and parallel to theline source of radiation to reflect a beam of radiation therefrom, asecond collimating system mounted for rotation with said specimen andcomprising a plurality of thin long sheets impervious to X- radiationwhich are mounted perpendicular to said axis of rotation, aGeiger-Muller tube for detecting said reflected beam of radiationmounted for rotation with said specimen and said second collimatingsystem whereby the reflected beam is detected and the intensity thereofdetermined, and a circular scale mounted perpendicular to the axis ofrotation and slideably secured to the Geiger-Muller tube to permitrelative movement of the latter over the scale for measuring angles ofrefiectionof the reflected beam from the specimen.

8. X-ray diffraction apparatus comprising an X-ray tube for generating aline source of X- radiaticn, a first collimating system comprising aplurality of thin long parallel sheets imper vious to X-radiationmounted perpendicular to line source to collimate the beam of radiationinto a plurality of parallel beams of radiation, means to mount aspecimen of crystalline material forr otation about a given axis passingtherethrough and parallel to the line source of radiation to reflect abeam of radiation therefrom, a second collimating system mounted forrotation with said specimen and comprising a plurality of thin longsheets impervious to X- radiation which are mounted perpendicular tosaid axis of rotation, means to limit the width of the collimatedreflected beam of radiation to source, a Geiger- Miiller tube fordetecting said reflected beam of radiation mounted for rotation withsaid specimen and said second collimating system whereby the reflectedbeam is detected and the intensity thereof determined, and a circularscale mounted perpendicular to the axis of rotation and slideablysecured to the Geiger-Muller tube to permit relative movement of thelatter over the scale for measuring angles of reflection of thereflected beam from the specimen.

' WILLIAM BARRISH.

EDV/ARD A. HAMACHER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,164,987 Bucky Dec. 21, 19151,589,833 Behnken et al June 22, 1926 2,011,540 Lee Aug. 13, 19352,331,586 Waisco Oct. 12, 1943 2,386,785 Friedman Oct. 16, 1945 FOREIGNPATENTS Number Country Date 670,322 Germany Jan. 16, 1939 OTHERREFERENCES Focusing X-Ray Monochromators by C. S. Smith, Review ofScientific Instruments, June 1941, pp. 312-314.

Scattering of X-Rays from Gases by E. 0. W01- lan, Physical Review, Apr.1931, pp.862-8'72.

A New Precision X-Ray Spectrometer by W. Soller, Physical Review, Aug.1924, pp. 158-167.

The New X-Ray Microscope by Gaylord Johnson, from the ScientificAmerican, May 1932, pp. 278-282.

